In order to achieve high circuit performance and density, MOSFET (metal-oxide-semiconductor field effect transistor) devices in silicon integrated circuit technology are scaled down to submicrometer range. In scaling down MOSFETs, the reduction of device dimensions is not accompanied by a corresponding reduction in power requirements. As a result, NMOS (n-channel MOS) devices are susceptible to channel hot-electron (CHE) instability. See Chenming Hu et al., "Hot-Electron-Induced MOSFET Degradation--Model, Monitor and Improvement," IEEE Transactions on Electron Devices, Vol. ED-32, No. 2 (Feb. 1985), pp. 375-385. The instability is caused by the very high electric field near the drain junction resulting from the short channel length and high supply voltage.
To alleviate the high electric field at the reduced MOSFET channel length, lightly doped drain (LDD) devices have been proposed. See K. Saito et al, "A New Short Channel MOSFET with Lightly Doped Drain," Denshi Tsushin Rengo Taikai (in Japanese) (April 1978), p. 220. In the LDD structure, narrow, self-aligned, n.sup.- regions are introduced between the channel and the n.sup.+ source/drain regions. The n.sup.- region spreads the high electric field out near the drain junction, allowing the device to be operated at a higher supply voltage with fewer hot-electron problems.
Several processes for fabricating lightly-doped drain field effect transistor (LDDFET) have been proposed. Spacer and overhang techniques are most commonly adopted. The spacer technique involves a reactive-ion etching (RIE) step after silicon dioxide is chemical vapor deposited to form sidewall oxide spacers. Oxide spacers are used to mask the heavy and deep implant of the n.sup.+ drain/source regions after the formation of the shallow n.sup.- drain/source regions. See FIG. 2, p. 590, of Paul J. Tsang et al., "Fabrication of High-Performance LDDFET's with Oxide Sidewall-Spacer Technology," IEEE Transactions on Electron Devices, Vol. ED-29, No. 4 (April 1982). The overhang technique involves a polysilicon over-etching step after SiO.sub.2 /Si.sub.3 N.sub.4 /poly-Si/SiO.sub.2 gate stack is patterned to form SiO.sub.2 /Si.sub.3 N.sub.4 overhangs. SiO.sub.2 /Si.sub.3 N.sub.4 overhangs are used to mask the heavy and deep implant of the n.sup.+ drain/source regions followed by the formation of the shallow n.sup.- drain/source regions. See FIG. 2, p. 1360, of Seiki Ogura et al., " Design and Characteristics of the Lightly Doped Drain-Source (LDD) Insulated Gate Field-Effect Transistor," IEEE Transactions on Electron Devices, Vol. ED-27, No. 8 (Aug. 1980).
Two alternative structures, buried LDD and graded/buried LDD structures, adopting sidewall oxide spacer technology, have also been demonstrated. See Ching-Yeu Wei et al., "Buried and Graded/Buried LDD Structures for Improved Hot-Electron Reliability," IEEE Electron Device Letters, Vol. EDL-7, No. 6 (June 1986), pp. 380-382. In the spacer technology, additional oxide deposition and oxide etch-back processes are needed. In the overhang technology, additional Si.sub.3 N.sub.4 /SiO.sub.2 deposition and polysilicon over-etching processes are required. In addition, these two processes are far too complicated for commercial application.
Still another method described is the so-called self-defined polysilicon sidewall (SEPOS) technique which uses SiO.sub.2 at the vertical sides of the polysilicon to define the oxide-framed polysilicon sidewall. See FIG. 1, p. 2463, of M. Saitoh, "Degradation Mechanism of Lightly Doped Drain (LDD) n-Channel MOSFET's Studied by Ultraviolet Light Irradiation," J. Electrochem. Soc.: Solid-State Science and Technology, Vol. 132, No. 10 (Oct. 1985), pp. 2463-2466.
Also proposed has been the self-aligned polysilicon source/drain (SAPSD) technique which uses a n.sup.+ polysilicon source/drain layer to allow the dopants to diffuse into the substrate to form the n.sup.- region. See FIG. 1, p. 314, of Tiao-Yuan Huang et al., "A MOS Transistor with Self-Aligned Polysilicon Source-Drain," IEEE Electron Device Letters, Vol. EDL-7, No. 5 (May 1986), pp. 314-316.
The inverse-T LDD (ITLDD) transistor has also been proposed. This transistor uses a sidewall oxide spacer to
define the n.sup.- region. See FIG. 2, page 743, of Tiao-Yuan Huang et al., "A Novel Submicron LDD Transistor with Inverse-T Gate Structure," IEDM 86 (International Electron and Device Meeting 1986), Sec. 31.7, pp. 742-745. Unfortunately, all of these techniques are too difficult because of the complexity of the processes.
In addition, a new LDD structure for NMOS FET has also been demonstrated. It may be made using a single ion implantation step to form n.sup.- regions due to the sloped sidewall of the structure. See FIG. 1, p. 28, of "A New Structure LDD for NMOSFET," Japan Semiconductor News, Vol. 3, No. 3 (June 1984), pp. 27-28. Unfortunately, the gate of the FET is much higher than source/drain regions.